Download Title: Fine-scale and Microhabitat Factors Influencing Terrestrial

Title: Fine-scale and Microhabitat Factors Influencing Terrestrial Amphibian Diversity in a
Low-elevation Old Growth Forest in Central Appalachia
Author:
Joseph A. Baecher
Eastern Kentucky University, Department of Biological Sciences
521 Lancaster Avenue
Moore Building, 308
Richmond, Kentucky, 40475
Project Narrative:
Introduction:
Observations of patterns in species richness along environmental gradients are
commonplace in ecology and biogeography, and a functional understanding of the mechanisms
influencing these patterns is crucial for preserving biodiversity (Gaston 2000, Willig et al. 2003).
Furthermore, analyzing species’ distribution and abundance along environmental gradients yields
invaluable information about their niche requirements, population dynamics, and biotic
interactions (Costa et al. 2008, Peterman and Semlitsch 2013), and can even inform decisions
about habitat management and restoration (Peterson 2006). However, unnatural environmental
gradients may not provide the spectrum of habitat parameters needed to fulfill the collective
niche requirements of local species due to a greater proportion of disturbed area, and therefore
research along natural gradients is needed to understand niche partitioning and model patterns in
biodiversity. Species’ distributions on the landscape, and thus, the landscape’s biodiversity, are a
function of these natural gradients, which include abiotic factors such as surface temperature,
moisture, relief, water and soil chemistry, and sunlight, and biotic factors such as vegetative
structure, and competition, and presence of predators, prey, and mates. Taxa likely to exhibit
patterns in biodiversity in response to such natural gradients are those with limited dispersal
capabilities, low reproductive success, and acute sensitivity to environmental conditions.
Indeed, one such group, amphibians, is particularly responsive to environmental gradients
(Werner et al. 2007, Semlitsch et al. 2015, Araújo et al. 2007). They are acutely sensitive to
environmental contaminates, water and soil chemistry, and dewatering because of their highly
permeable skin (Boone et al. 2007, Willson et al. 2012, Walls et al. 2013). As ecotherms,
amphibians rely on landscape structure (Cowles 1958), and thermal and hydrologic regimes
(Semlitsch et al. 2015) as well as prey availability, making them especially sensitive to habitat
destruction and degradation (Brooks et al. 2002). As such, amphibians serve as effective
biological indicators (Welsh and Ollivier 1998). These characteristics, in combination with a host
of pressures, namely disease and habitat destruction, are why amphibians are currently
experiencing unprecedented population declines at a global scale (Houlahan et al. 2000, Stuart et
al. 2004).
Despite amphibians’ sensitivity to the environment, some amphibians, particularly
terrestrial plethodontid salamanders, can be found in extraordinary abundance (up to 7.38
individuals/m2) in natural systems (Burton and Likens 1975a; Semlitsch et al. 2014). As a result
of their abundance, and an extensive list of known vertebrate predators, they have tremendous
influence on direction and magnitude of energy flow through food webs (Burton and Likens
1975b; Semlitsch et al. 2014). Wyman (1998) found, experimentally, that through predation of
macroinvertebrates, presence of a terrestrial salamander, Plethodon cinereus (Eastern red-backed
salamander), can indirectly reduce decomposition of leaf-litter on the forest floor by 11–17%,
with implications in regional biogeochemistry and global carbon cycling. Terrestrial salamanders
are a vital component in temperate forest ecosystems of the eastern US, and mesic forests of the
Appalachian Mountains contain the greatest diversity of caudates in the world (Dodd 2004). It is,
therefore, crucial to understand the relationship of terrestrial amphibians to natural
environmental gradients in Appalachia.
Accurately describing details of terrestrial salamander distributions in the forests of
Appalachia has long been a goal of scientists and naturalists (Cope 1870, Brimely 1912, King
1939, Hairston 1949, Highton and Peabody 2000, Dodd 2004). These observations are
paramount to our knowledge of terrestrial amphibian natural history; however, further
investigations are needed to understand the fine-scale relationships of these species within
landscape structure. Recent studies have focused on the influence of anthropogenic disturbance
regimes (e.g. silviculture [Pentranka et al. 1993], urbanization [Scheffers and Paszkowski 2012],
habitat fragmentation [Wyman 1990], and extraction of natural resources [Drohan et al. 2012])
on terrestrial amphibian ecology in Appalachia; however, few have examined what governs their
distribution in the sparsely distributed mature central Appalachian forests. Previous studies
across natural ecosystems of Appalachia postulated that climatic variables likely delimit
distributions of high-elevation specialists, including P. jordani (Red-cheeked Salamander), P.
metcalfi (Southern Gray-cheeked Salamander), P. shermani (Red-legged Salamander), P.
yonahlossee (Yonahlossee Salamander; Hairston 1950, 1951), and Desmognathus ocoee (Ocoee
Salamander; Ford et al. 2002), and interspecific competition likely limits upward dispersal in
lower elevation plethodontids, like P. glutinosis (Eastern slimy salamander). Notwithstanding,
distributions of lower elevation terrestrial salamanders in Appalachia are thought to be
influenced chiefly by availability of microhabitat features.
Previous studies of amphibian communities in undisturbed Appalachian forests have
determined that microhabitat parameters like soil chemistry (Jaeger 1971a, Wyman 1988),
abundance of natural cover (i. e. coarse woody debris [CWD], rocks, and leaf litter; McKenny et
al. 2006), and forest composition/canopy structure (Gibbs 1998) greatly influence terrestrial
salamander distribution and abundance. Wyman and Jancola (1992) found that terrestrial
salamander abundance and species richness increased steeply with soil pH from 3.5 to 4.5, and
that community composition is greatly influenced by soil pH due to species-specific acid
tolerance (Wyman 1988). Wyman and Hawksley-Lescault (1987) showed that P. cinereus
tolerates a narrow range of soil pH, and that over 25% of the forest floor sampled contained
unsuitable acid concentrations, and Eurycea bislineata (Northern Two-lined Salamander), D.
fuscus (Northern Dusky Salamander), and Ambystoma maculatum (Spotted Salamander)
tolerated more alkaline conditions, and were found in a wider range of pH concentrations.
Similarly, Wyman (1988) found soil moisture to limit the distribution and abundance of
Lithobates sylvaticus (Wood Frog), Notophthalmus viridescens (Eastern Newt), E. bislineata, D.
fuscus, and A. maculatum. Other microhabitat features like substrate, cover type, and canopy
have been shown to have marked affects on terrestrial amphibian populations. Peterman and
Semlitsch (2013) found that dense-canopy ravine habitats with high moisture, and low solar
transmittance conferred greatest abundance of P. glutinosis. Leaf-litter depth (Menin et al. 2007)
and coarse woody debris on the forest floor has been shown to be important habitat features for
numerous species of terrestrial salamanders across the eastern US (Harpole and Hass 1999, Todd
and Rothermel 2006, Semlitsch et al. 2008). Furthermore, exploitation of preferred microhabitat
by terrestrial salamanders may not be possible in the presence of competition (Jaeger 1971b) or
predation (Hairston 1986), and therefore the niche structure and distribution of species may vary
significantly (Hairston 1980).
Variation in species-specific responses to environmental gradients is influenced by
community interactions (MacArthur and Levins 1967), particularly in species-packed
communities with overlap in fundamental niches (MacAuthor 1970). Plethodontid salamanders
have long been recognized as model systems for the study of community interaction (Hairston
1949, 1951, 1980 and Highton 1972); up to five species can be found in a single habitat (Highton
1995). Microhabitat selection, resource allocation, and niche structures in salamanders belonging
to the genus Desmognathus have been shown to vary greatly by community structure, and even
in the presence of congeners of varying size (Krzyik 1979, Keen 1982). The effects of species
interactions on the distribution of terrestrial salamanders of the genus Plethodon in the eastern
United States is particularly fascinating. Intraspecific competition often leads to similarly sized
species replacing one another in some regions (Jaeger 1970), and interspecific competition for
food and space causing horizontal and vertical stratification in species distributions (Hairston
1951, Adams and Rohlf 2000).
The ecology of terrestrial salamander communities in lower elevation Appalachian forest,
like those of Central Appalachia, have not been studied as thoroughly as regions with greater
topographic relief and a higher proportion of state and national parks (Piedmont, Blue Ridges,
Southwestern Appalachians). Due to the diversity and endemism of terrestrial salamanders,
community structure varies dramatically across physiographic regions of Appalachia, and
therefore species interactions are likely unpredictable. Such phenomena can have tremendous
influence on how terrestrial salamanders utilize environmental and resource gradients, and,
therefore, regional studies are greatly needed.
The objectives of this study are to determine how geographic (aspect, slope, and
elevation) and environmental (coarse woody debris, canopy openness, and soil moisture,
temperature, and pH) variables, and the co-occurrence of species, shape the fine-scale
distribution and abundance of terrestrial amphibians in low elevation forest of Central
Appalachia.
Methods:
Study Area:
The Lilley Cornett Woods (LCW) Appalachian Ecological Research Station (Letcher
County, KY, USA) is a 223-ha tract of land in the Cumberland Plateau. LCW has a temperate
humid continental climate, with mean annual precipitation = 1130 mm and annual mean
temperature = 13°C (Hill 1976). My study will take place in Short Trail Stand (STS), a mixed
mesophytic old growth stand of LCW (Braun 1950; Martin 1975; Figure 1). STS contains 57
circular 0.04-ha sample plots, originally established by Martin (1975), representing
approximately 10% of the total upland area in STS. Sample plots were created along vertical
ravines (concave slope shape) and ridges (convex slope shape) for each slope position (lower
[<365 m], middle [365 m – 427 m], upper, and ridge [> 1480 m]). Two sampling events will
occur during three seasons (spring, summer, and fall) of 2016 at all 57 sites, resulting in six
replications at each site (two per season). A subset of the 57 sites will be selected for an
additional sampling event (three per season) each season to calculate probability of detection for
terrestrial salamander species in STS. These sample plots will be chosen using vertically
stratified random sampling by dividing STS into four elevational strata (lower, middle, upper,
and ridge) and randomly selecting six sites with north- and south-facing slopes from each
stratum. During every sampling event amphibian surveys, including visual encounter and quadrat
sampling, will be conducted at each of the 57 plots (see below for details).
Field Surveys:
To eliminate sampling bias, increase replicability, and ensure plots are sampled
thoroughly, a simple random sample design will be used, wherein the direction of linear visual
encounter survey (VES) transects at each plot are determined by randomly selecting a bearing
between 0° and 180°, with the midpoint of all transects pivoting and centering on the plot
marker, which is positioned at the geometric center of each sample plot (Martin 1975).
Microhabitat type may potentially differ across my sampling areas, therefore a transect design
was selected to most thoroughly assess amphibian species within mid-sized sampling plots
(Jaeger and Inger 1994). A 50x3-m transect will be established and searched opportunistically
for amphibians under all natural cover (rocks and woody debris; Bailey et al. 2004). Visual
encounter surveys will be conducted at every site during the spring, summer, and fall of 2016–
2017 (n=6), during times of day most advantageous for encountering amphibians (0800 – 1100
hrs and 1500 – 1900 hrs. Data collected at time of capture (i.e. time of day, temperature, days
since last rain, Julian date) will be incorporated into models to control for variation in conditions
among sampling periods, locations, and seasons.
Quadrat sampling:
With goals of sampling multiple species, including some large-bodied species (e.g.,
Pseudotriton ruber (Red Salamander), P. glutinosis, and A. maculatum), utilizing a variety of
habitat types, a broad-quadrat sampling (large quadrats; 8m x 8m) technique would be most
appropriate for this study (Jaeger and Inger 1994). However, thoroughly sampling and
classifying habitat in such a large area may prove infeasible, therefore an enlarged pointsampling technique is posed, in which all leaf-litter and natural cover within a randomly placed
4-m2 quadrat are removed and searched for amphibians. Quadrat sampling will occur in
conjunction with visual encounter surveys. Quadrat placement within the linear VES transect
will be determined by selecting a random number from an interval (1–50) corresponding to the
length of the VES transect; the quadrat will then be tossed at the randomly selected distance
along the transect.
Amphibian data collection:
Snout-to-vent length (SVL) will be measured in situ by placing the animal in a clear
plastic bag, moistened with distilled H2O, and measuring from the tip of the snout to the
posterior edge of the vent (nearest 1 mm) using a hard ruler fitted with a 90° stop. Mass will be
recorded using a digital PESOLA scale (accuracy = 0.1 g). Juveniles and adults will be
delineated to the best of our ability using published species-specific SVL and masses (Houck
1982, Johnson 2000, Dodd 2004, Trauth et al. 2004, Niemiller and Reynolds 2011). Additional
demographic data (sex, gravidity, damage/condition) will be collected in situ before releasing
animals at their capture location.
Environmental Data Collection:
Quantification of forest canopy openness will be achieved using hemispherical canopy
photography (Frazer et al. 1997; Baldwin et al. 2006), wherein a 180° fish-eye lens is used to
capture canopy structure (Herbert 1987), as well as canopy light transmission. Once per season,
canopy of each sample plot will be photographed during the early morning (0600 – 0700 hours).
Photographs will then be analyzed using Gap Light Analyzer (GLA) ver. 2 (Frazer et al. 1999).
Each season, fallen coarse woody debris (CWD; logs and branches; excluding snags)
larger than 20 cm in diameter (Miller and Liu 1991) will be measured at each 50x3-m VES
transect. CWD measurements will follow methodology of Muller (2003) and Davis et al. (2015),
wherein terminal and center diameters of each item are measured using meter tape. CWD
volume is calculated using the formula of a truncated cone (or conical frustum):
𝝅
𝑽 = 𝟑 𝒉(𝑹𝟐 + 𝑹𝒓 + 𝒓𝟐 );
where h = length of the item, R = radius at the terminal of greater diameter, and r = radius at the
terminal of smaller diameter.
During each sampling event leaf-litter density will be calculated by dividing each 4-m2
quadrat into 4 1-m2 subplots, and measuring leaf-litter depth and wet-mass in each. Depth will be
measured as distance from the leaf-litter surface (at a point representative of the subplot) to the
top soil. Density will be measured by collecting all leaf-litter within a subplot into a bag and
weighing using a PESOLA scale. Thus, leaf-litter density will be reported as an average of 4
measurements at each quadrat per sampling event.
Soil moisture and temperature will be measured during each quadrat-sampling event
using a Fieldscout TDR 100 Soil Moisture Meter (Spectrum Technologies, Inc.) and a
GENERAL 8” digital thermometer (accuracy = ±1° C), respectively. Measurements will be taken
at 4 locations within each 4-m2 quadrate and averaged. Soil pH, measured as the upper-most
horizon below leaf-litter, will be recorded in situ at each sampling quadrat using a YSI 556
multi-parameter water quality meter by creating a 1:1 soil-to-water suspension, stirring
vigorously, and allowing to stand for 30 minutes before measuring (YSI Inc. 2015).
GIS and Spatial Covariates:
Aspect and elevation of each plot will be collected using ArcGIS 10.1 (ESRI 2011).
Elevation data will be ground-truthed in situ for accuracy and validation of GIS data. Geospatial
and elevational data will be ground-truthed in late-winter 2016, prior to site selection. The slope
of each plot will be measured using a clinometer.
Data Analysis:
Amphibian diversity modeling:
Amphibian count data will be used to calculate species richness and evenness, and
biological diversity indices (Simpson and Shannon-Wiener) for each site (n=60). Geospatial data
(aspect, slope, and elevation), and environmental data (soil pH, moisture, and temperature;
CWD, leaf-litter density, and canopy openness) will then be used in regression models as
predictor variables for amphibian diversity metrics using SPSS 22 (IBM SPSS Statistics 2013).
Akaike’s Information Criterion (AIC) will be used to select models and determine which spatial
and microhabitat factors best explain variation in amphibian diversity metrics and spatial
distribution (Mazerolle 2006).
Occupancy modeling:
To account for imperfect detection probabilities (p < 1), a binomial-mixture model for
spatially replicated sampling designs (Royle 2004) will be used to analyze count data and
estimate terrestrial amphibian abundances as a function of site-level (i.e. aspect, slope, and
elevation) and environmental (i.e. soil chemistry, moisture, and temperature; CWD, leaf-litter
depth, and canopy structure) covariates. Because detection probabilities will likely vary by
species and life-stage, species-exclusive models will be generated, and run separately for
juveniles and adults. A noteworthy assumption of this model is that populations are closed
during sampling events (i.e. no immigration, emigration, births, or deaths). Fortunately,
horizontal migration in terrestrial plethodontid salamanders is limited due to small home ranges
(Kleeberger and Werner 1982), long-term site fidelity (Marvin 2001), and life histories that do
not require seasonal migration to aquatic habitat for reproduction. Furthermore, estimates of
daily movement in P. cinereus, a common, widely spread, terrestrial plethodontid in eastern US
(Conant and Collins 1998), suggests that horizontal immigration and emigration in my study’s
0.04-ha sampling plots is unlikely (Ousterhout and Liebgold 2010), and perhaps only possible in
peripheral subplots. Vertical migration is known to affect detection probabilities in terrestrial
amphibians, and is largely driven by climatic conditions and soil moisture; therefore, including
such variables as Julian date, soil temperature, soil moisture, and days since last rain in the
models should account for variability in detection (Peterman and Semlitsch 2013). Resulting
candidate models will be weighed and a top predictive model selected.
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Figures:
Fig. 1: Map of Lilley Cornett Woods (LCW) Appalachian Ecological Research Station in
Letcher County, Kentucky, USA. Short Trail Stand (STS), a mixed mesophytic old growth stand
belonging to LCW, is delineated in red (dashed).